add/remove class without jQuery (just Vanilla JS)

add_remove_class.js

  /**
   * Remove a CSS class from an HTML element.
   * @param {HTMLElement} el The HTML element to modify.
   * @param {string} className The class to remove.
   * @returns null
   * @see http://youmightnotneedjquery.com/#remove_class
   */
  function removeClass(el, className) {
    if (el.classList)
      el.classList.remove(className);
    else
      el.className = el.className.replace(new RegExp('(^|\\b)' + className.split(' ').join('|') + '(\\b|$)', 'gi'), ' ');
  }

  /**
   * Add a CSS class to an HTML element.
   * @param {HTMLElement} el The HTML element to modify.
   * @param {string} className The class to add.
   * @returns null
   * @see http://youmightnotneedjquery.com/#add_class
   */
  function addClass(el, classsName) {
    if (el.classList)
      el.classList.add(className);
    else
      el.className += ' ' + className;
  }